The plan, funded in part by Jacobs, would have added a bypass bridge to the Cabrillo Bridge on the west and removed cars and traffic from the center of the park, including the main Plaza de Panama, to make it a pedestrian-oriented gathering space.

But the Save Our Heritage Organisation, which sued the city and Jacobs’ Plaza de Panama Committee, objected to the bypass because of its visual impact on the historic park entrance. It cited support from other national and state agencies and organizations.

The city or Jacobs’ Plaza de Panama Committee could appeal the ruling, or Jacobs and SOHO could negotiate a compromise, but time is running short to make major changes before the January 2015 start of the centennial of the 1915 Panama-California Exposition, which made Balboa Park what it is today.

SOHO called an appeal “unwise,” and Jacobs declined any comment at least until today. He has funded $8 million so far and had committed to raising $30 million privately to match $15 million from the city. He had said earlier he might support an alternate plan that met his goals, but none emerged from an exhaustive environmental review process that the judge upheld as sufficiently thorough.

In a statement, Mayor Bob Filner called on SOHO, Jacobs and the committee planning the 2015 centennial celebration of the exposition to “work cooperatively and expeditiously” to turn the Plaza de Panama into “predominantly pedestrian use.”

“With good faith and hard work on each side, a mutually acceptable vision can be agreed upon quickly,” said Filner, who during his campaign last year had opposed the Jacobs plan.

In essence, City Attorney Jan Goldsmith’s office argued that parking and traffic in the park’s center is bad and that Jacobs’ plan would improve the experience for visitors. The judge agreed that the plan was laudable, but parking and traffic, even if an undesirable use of parkland is still a reasonable use of the land — and that is the legal standard he has to follow when altering a historic site such as Balboa Park is involved.

“The city abused its decision in making the finding of ‘no reasonable beneficial use of the property,’ ” Taylor said in his final ruling, “and SOHO is entitled to a writ of mandamus directing the city to set aside this determination and set aside its approval” of the Jacobs plan.

The approval included amendments to the park’s master plan, the design of the bypass bridge, conversion of the plaza parking lot into a pedestrian zone and construction of an 800-space paid parking garage south of the Spreckels Organ Pavilion.

Goldsmith’s office issued a statement welcoming the judge’s upholding of the city’s environmental analysis of the plan but did not comment on the ruling overturning the Jacobs plan, saying it will first confer with the City Council on the next course of action.

SOHO said it was “ecstatic” about the ruling.

“The plan would have caused significant, irreparable and irreversible harm to Balboa Park’s historic structures, its environment, its canyons and roadways,” SOHO Executive Director Bruce Coons said in a statement.

Coons said in an interview that he and others offered many alternatives that would have removed parking from the Plaza de Panama but allowed limited car access.

“We approached them (Jacobs’ committee) with a million different ideas. We’ve been trying to compromise for 2½ years.”

Jacobs had hoped the improvements would be ready in time for the January 2015 kickoff to the yearlong expo centennial celebration.

What happens next is unknown.

Last week, in response to a tentative ruling that indicated the judge’s thinking, Filner said if no parking in the plaza is the sole goal, he could accomplish that by posting six traffic cones in the plaza. But his preference is to carry out the park’s present master plan and operate Disneyland-like trams to transport visitors from distant garages.

Coons conceded that “anything’s possible” when it comes to a new round of mediation.

However, he seemed to throw cold water on any talk of making up with Jacobs, with whom he’d met privately to review options.

“Today’s victory should be seen as a lesson to never allow plutocratic interests to override the wishes of the public,” Coons said in his statement.

Filner was more conciliatory, saying, “The opposing parties in this issue have each shared a common and compelling goal: to beautify Balboa Park and make it more pedestrian friendly.”

However, time is of the essence if major changes to the plaza are to be completed by January 2015.

Even if no appeal is filed, an alternate plan could take too long to implement, especially if a new environmental impact report is required.

The Balboa Park Committee, the official body that oversees the park, is scheduled to discuss the judge’s ruling at its 6 p.m. Thursday meeting at the Balboa Park Club.

Committee Chairman David Kinney said he still hopes Jacobs and SOHO will meet and work out a compromise.

“I’m not disappointed yet, because I’m still hoping something can be done,” Kinney said. “If nothing can get done — and I think that’s a long way off — yes, I will be disappointed.”